xxviii Introduction to the Makerstoun Observations, 1845 and 1846. 



whence 



aX_2M>'1 p_ y\ 

 X '^Xr''^?^'?) ^-^ 



If the deflecting bar be now employed to deflect a freely-suspended unifilar magnet, 



M 



in order to determine the value of — , as in the ordinary observations for absolute 



horizontal intensity ; u being the angle of deflection for a distance r^ we have 



r r 



If the bifilar and unifilar bars are of the same dimensions jo and q, which are quan- 

 tities depending upon the distribution of the magnetism in the bars, may be consi- 

 dered equal to pi and q^-, and if the deflections for both bars be made at the same 

 distances, or r=ri then 



aX ^ 

 — - = tan u, 

 X 



and kJ^^ (3.) 



n 



If, however, the bifilar and unifilar magnets are of dififerent dimensions, the value 



of — — should be obtained from the deflections of the unifilar at difi'erent distances, 

 X 



jt?i and qx being eliminated ; that value being substituted in equation (2.), and deflec- 

 tions of the bifilar being obtained for difi'erent values of r, /> and q also may be 

 eliminated. 



33. Wooden beams having been placed in the prolongations of the bifilar mag- 

 net, and at right angles to these, lines were drawn upon them, passing through the 

 centre of the magnet, — one in the magnetic meridian, the other at right angles to 

 it ; several distances from the centre of the suspended magnet were marked oflF on 

 each side with a beam compass ; a similar structure was erected for the declinome- 

 ter. The followinaj observations were then made : — 



34. Is^, A cylindrical magnet, 3'65 inches long, was employed to deflect the bifilar 

 and declinometer magnets; these two magnets are of the same dimensions, 15 inches 

 long, and were obtained at the same time from the same maker. The short de- 

 flecting bar was placed at different distances to the east, and at the same distances 

 to the west, of the bifilar bar, and the deflections of the bifilar were observed in 

 scale divisions. Observations of deflection of the declinometer magnet were then 

 obtained with the same deflector — the deflector, however, being placed at the same 

 distances, as in the other case, to the north and south of the declinometer magnet : 

 in both cases, the prolongation of the suspended bar, in its normal position, passes 

 through the centre of the deflector. The results are obtained in the 1st portion 

 of Table 3. 



